There can be several reasons for this. It appears that you have installed the SynoCommunity package. The DS418play has the Apollolake platform and this platform is still supported for the v1.4.0-19, but no longer for the v1.7.1-20:
It seems that you already have v1.7.1-20 internally, but the basis is v1.4.0-19 and this is one of the basic disadvantages of that package concept. Therefore I strongly recommend installing the Syncthing package, because updates always update the entire package:
With that package, also your platform is supported:
Maybe you give some details about that all.